Exported Blender UV Map is blurry
PNG will be anti-alliased.

Try to export your UV map as .SVG. It will be anti aliased too but you can change this. 


I'm using gimp (it's free). I don't know if you can do that in paint.net

Open your .SVG in gimp. Choose the size and check "Import Paths" and "merge imported Paths" (tried to translate).



in the "Paths" tab, click on "paint along path"



uncheck "antialiasing", choose your line width (color will be foreground color)



your lines will be exactly the color you want, nothing more, nothing less.